I gave my cat the over the counter Hartz flea medicine. The kind you rub on their neck to get rid of the fleas (Like Advantage or Frontline) and she got VERY VERY ill. She was shaking, sweating and extremely lethargic. She had an extremely high fever and was soaking wet with perspiration.
Fortunately I noticed this right away (she came to me and “showed” me how she was feeling) and I washed off as much as I could on her neck area and kept an eye on her for the next few hours.
After washing it off and having her drink lots of water, she improved and I ended up not needing to take her to the vet.
But she was really sick for about 6 hours. Hartz is evil! I’m so thankful that I was observant of her change in mood and that she survived.
I now use diatomaceous earth to keep fleas away.